At present, a lead-acid battery is installed in many vehicles. This lead-acid battery supplies power to a starter motor, or many kinds of electric devices. The lead-acid battery is inexpensive, but has the characteristics of a short cycle life, compared with a nickel hydride storage battery or a lithium ion storage battery. In the vehicles having the idle stop function (idle reduction function), as the number of charging and discharging is large, especially the life of the lead-acid battery becomes short.
Then, a configuration in which the lead-acid battery, and the nickel hydride storage battery or the lithium ion storage battery are connected in parallel is proposed. In this parallel circuit, it is proposed that a switch between the lead-acid battery and the nickel hydride storage battery or the lithium ion storage battery is provided (for example, refer to patent literature 1). Mainly, this switch is turned off when power is supplied from the lead-acid battery to the starter motor, and is used in order to stabilize a voltage supplied from the nickel hydride storage battery or the lithium ion storage battery to the electric devices. Therefore, in a normal state, this switch is ON in principle.